Process for fabricating circuit assemblies using electrodepositable dielectric coating compositions

ABSTRACT

Provided is a process for forming metallized vias in a substrate including the steps of (I) applying to an electroconductive substrate an electrodepositable coating composition onto all exposed surfaces of the substrate to form a conformal dielectric coating; (II) ablating a surface of the dielectric coating to expose a section of the substrate; (III) applying a layer of metal to all surfaces to form metallized vias in the substrate. Also disclosed are processes for fabricating a circuit assembly which include the application of an electrodoepositable coating composition onto exposed surfaces of the substrate/core to form a conformal dielectric coating thereon. The electrodepositable coating composition includes a resinous phase dispersed in an aqueous phase, where the resinous phase has a covalently bonded halogen content of at least 1 percent by weight. The dielectric coating derived therefrom has a low dielectric constant and low dielectric loss factor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 09/802,001, filed Mar. 8, 2001; now abandoned U.S. patentapplication Ser. No. 09/851 904 filed May 9, 2001; now abandoned andU.S. patent application Ser. No. 09/901.373, filed Jul. 9, 2001 now U.S.Pat. No. 6,671,950 each of which is incorporated in its entirety hereinby reference. Also this application is related to U.S. patentapplication Ser. No. 10/184,195 now U.S. Pat. No. 6,713,587; U.S. patentapplication Ser. No. 10/184,387; and U.S. patent application Ser. No.10/183.674 now U.S. Pat. No. 6,824959, all filed concurrently herewith.

FIELD OF THE INVENTION

The present invention relates to processes for forming metallized viasand for fabricating multi-layer circuit assemblies comprising adielectric coating, particularly a dielectric coating applied byelectrodeposition.

BACKGROUND OF THE INVENTION

Electrical components, for example, resistors, transistors, andcapacitors, are commonly mounted on circuit panel structures such asprinted circuit boards. Circuit panels ordinarily include a generallyflat sheet of dielectric material with electrical conductors disposed ona major, flat surface of the sheet, or on both major surfaces. Theconductors are commonly formed from metallic materials such as copperand serve to interconnect the electrical components mounted to theboard. Where the conductors are disposed on both major surfaces of thepanel, the panel may have via conductors extending through holes (or“through vias”) in the dielectric layer so as to interconnect theconductors on opposite surfaces. Multi-layer circuit panel assemblieshave been made heretofore which incorporate multiple stacked circuitpanels with additional layers of dielectric materials separating theconductors on mutually facing surfaces of adjacent panels in the stack.These multi-layer assemblies ordinarily incorporate interconnectionsextending between the conductors on the various circuit panels in thestack as necessary to provide the required electrical interconnections.

In microelectronic circuit packages, circuits and units are prepared inpackaging levels of increasing scale. Generally, the smallest scalepackaging levels are typically semiconductor chips housing multiplemicrocircuits and/or other components. Such chips are usually made fromceramics, silicon, and the like. Intermediate package levels (i.e.,“chip carriers”) comprising multi-layer substrates may have attachedthereto a plurality of small-scale chips housing many microelectroniccircuits. Likewise, these intermediate package levels themselves can beattached to larger scale circuit cards, motherboards, and the like. Theintermediate package levels serve several purposes in the overallcircuit assembly including structural support, transitional integrationof the smaller scale microcircuits and circuits to larger scale boards,and the dissipation of heat from the circuit assembly. Substrates usedin conventional intermediate package levels have included a variety ofmaterials, for example, ceramic, fiberglass reinforced polyepoxides, andpolyimides.

The aforementioned substrates, while offering sufficient rigidity toprovide structural support to the circuit assembly, typically havethermal coefficients of expansion much different than that of themicroelectronic chips being attached thereto. As a result, failure ofthe circuit assembly after repeated use is a risk due to failure ofadhesive joints between the layers of the assembly.

Likewise, dielectric materials used on the substrates must meet severalrequirements, including conformality, flame resistance, and compatiblethermal expansion properties. Conventional dielectric materials include,for example, polyimides, polyepoxides, phenolics, and fluorocarbons.These polymeric dielectrics typically have thermal coefficients ofexpansion much higher than that of the adjacent layers.

There has been an increasing need for circuit panel structures whichprovide high density, complex interconnections. Such a need can beaddressed by multi-layer circuit panel structures, however, thefabrication of such multi-layer circuit assemblies has presented seriousdrawbacks.

Generally multi-layer panels are made by providing individual, dualsided circuit panels with appropriate conductors thereon. The panels arethen laminated one atop the other with one or more layers of uncured orpartially cured dielectric material, commonly referred to as “prepregs”disposed between each pair of adjacent panels. Such a stack ordinarilyis cured under heat and pressure to form a unitary mass. After curing,holes typically are drilled through the stack at locations whereelectrical connections between different boards are desired. Theresulting holes or “through vias” are then coated or filled withelectrically conductive materials usually by plating the interiors ofthe holes to form a plated through via. It is difficult to drill holeswith a high ratio of depth to diameter, thus the holes used in suchassemblies must be relatively large and consume a great deal of space inthe assembly.

U.S. Pat. No. 6,266,874 B1 discloses of method of making amicroelectronic component by providing a conductive substrate or “core”;providing a resist at selected locations on the conductive core; andelectrophoretically depositing an uncured dielectric material on theconductive core except at locations covered by the resist. The referencesuggests that the electrophoretically deposited material can be acationic acrylic- or cationic epoxy-based composition as those known inthe art and commercially available. The electrophoretically depositedmaterial then is cured to form a conformal dielectric layer, and theresist is removed so that the dielectric layer has openings extending tothe conductive core at locations which had been covered by the resist.The holes thus formed and extending to the coated substrate or “core”are commonly referred to as “blind vias”. In one embodiment, thestructural conductive element is a metal sheet containing continuousthrough holes or “through vias” extending from one major surface to theopposite major surface. When the dielectric material is appliedelectrophoretically, the dielectric material is deposited at a uniformthickness onto the conductive element surface and the hole walls. It hasbeen found, however, that the electrophoretically deposited dielectricmaterials suggested by this reference can be flammable, and thus do notmeet typical flame retardancy requirements.

U.S. Pat. Nos. 5,224,265 and 5,232,548 disclose methods of fabricatingmulti-layer thin-film wiring structures for use in circuit assemblies.The dielectric applied to the core substrate preferably is a fully curedand annealed thermoplastic polymer such as polytetrafluoroethylene,polysulfone, or polyimide-siloxane, preferably applied by lamination.

U.S. Pat. No. 5,153,986 discloses a method of fabricating metal corelayers for a multi-layer circuit board. Suitable dielectrics includevapor-depositable conformal polymeric coatings. The method usesperforate solid metal cores and the reference describes generallycircuitization of the substrate.

U.S. Pat. No. 4,601,916 suggests that while electrodeposition of aninsulating coating directly to the metal wall portions of the holes cancreate a uniform film of resin on the hole walls without producingthinning of the coating at the top and bottom rims of the holes, thesubsequent metal deposits would not adhere to the hole walls and,further, that the electrical insulating properties were inadequate.Hence, the reference is directed to an improved method for formingplated through holes in metal core printed circuit boards byelectrophoretically depositing coatings thereon which comprise anelectrodepositable resinous coating including a solid inorganic fillerin finely divided form. Suitable fillers include clays, silica, alumina,silicates, earths and the like. The composition exhibits a volumeresistivity greater than 10⁴ megohm-cm between the printed circuitconductor and the metal core. The method comprises electrophoreticallydepositing the aforementioned composition onto the metal wall portionsof the holes; curing the resinous coating, the thickness of which beingat least 0.025 millimeters; creating a hydrophilic microetched surfaceon the coating with an aqueous oxidizing solution to promote adhesion;depositing a metal layer on the surface of the resinous coating on thehole walls and on the insulating surface layers, the metal layeradhering to the coating with a specified peel strength, and forming aprinted circuit on the insulated metal substrate by standard printedcircuit techniques.

U.S. Pat. No. 4,238,385 discloses coating compositions forelectrophoretic application to electroconductive substrates for printedcircuits. The compositions comprise a pigment-containing finely dividedsynthetic resin powder where the resin includes an epoxy resin and thepigment includes 2 to 10 weight parts of a finely-divided silica,admixed with a cationic resin. The composition forms an insulating filmon the electroconductive substrate which is suitable for printedcircuits providing desirable properties such as dimensional stabilityand mechanical strength.

Circuitization of intermediate package levels is conventionallyperformed by applying a positive- or negative-acting photoresist(hereinafter collectively referred to as “resist”) to the metallizedsubstrate, followed by exposure, development, etching and stripping toyield a desired circuit pattern. Resist compositions typically areapplied by laminating, spray, or immersion techniques. The resist layerthus applied can have a thickness of 5 to 50 microns.

In addition to the substrates previously mentioned, conventionalsubstrates for intermediate package levels can further include solidmetal sheets such as those disclosed in U.S. Pat. No. 5,153,986. Thesesolid structures must be perforated during fabrication of the circuitassembly to provide through vias for alignment purposes.

In view of the prior art processes, there remains a need in the art formultilayer circuit panel structures which provide high density andcomplex interconnections, the fabrication of which overcomes thedrawbacks of the prior art circuit assemblies.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a process forforming metallized vias in a substrate. The process comprises the stepsof: (I) electrophoretically applying to an electroconductive substratean electrodepositable coating composition onto all exposed surfaces ofthe substrate to form a conformal dielectric coating thereon, theelectrodepositable coating composition comprising a resinous phasedispersed in an aqueous phase, the resinous phase comprising: (a) anungelled, active hydrogen-containing, ionic group containing resin, and(b) a curing agent reactive with the active hydrogens of the resin (a),the resinous phase having a covalently bonded halogen content of atleast 1 percent by weight based on total weight of resin solids presentin the resinous phase; (II) ablating a surface of the conformaldielectric coating in a predetermined pattern to expose a section of thesubstrate; and (III) applying a layer of metal to all surfaces to formmetallized vias in the substrate.

In another embodiment, the present invention relates to a process forfabricating a circuit assembly comprising the steps of: (I) providing anelectroconductive core; (II) applying electrophoretically theelectrodoepositable coating composition described above onto all exposedsurfaces of the core to form a conformal dielectric coating thereon;(III) ablating a surface of the conformal dielectric coating in apredetermined pattern to expose a section of the core; (IV) applying alayer of metal to all surfaces to form metallized vias in the core; and(V) applying a resinous photosensitive layer to the metal layer.

The present invention also is directed to a process for fabricating acircuit assembly comprising the steps of: (I) providing anelectroconductive core; (II) providing a photoresist at predeterminedlocations on the surface of the core; (III) applying electrophoreticallyan electrodepositable coating composition described above over the coreof step (II), wherein the coating composition is depositedelectrophoretically over all surfaces of the core except at thelocations having the photoresist thereon; (IV) curing theelectrophoretically applied coating composition to form a curedconformal dielectric layer over all surfaces of the core except at thelocations having the photoresist thereon; (V) removing the photoresistto form a circuit assembly having vias extending to the core at thelocations previously covered with the resist; and (VI) optionally,applying a layer of metal to all surfaces of the circuit assembly ofstep (V) to form metallized vias extending to the core.

The present invention is further directed to a substrate and circuitassemblies coated by the respective aforementioned processes.

DETAILED DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients, reaction conditions and soforth used in the specification and claims are to be understood as beingmodified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical values, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between andincluding the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

As previously mentioned, in one embodiment, the present invention isdirected to a process for forming metallized vias in a substrate, theprocess comprising: (I) electrophoretically applying to anelectroconductive substrate an electrodepositable coating composition(described in detail below) onto all surfaces of the substrate to form aconformal dielectric coating thereon; (II) ablating a surface of thedielectric coating in a predetermined pattern to expose a section of thesubstrate; and (III) applying a layer of metal to all surfaces of thesubstrate of step (II) to form metallized vias in the substrate.Optionally, the process further includes step (IV) applying aphotosensitive layer, as described below, to the metal layer.

In further embodiments, the present invention is directed to processesfor fabricating multi-layer circuit assemblies. In one embodiment, thepresent invention is directed to a process for fabricating a circuitassembly comprising the steps of: (I) providing an electroconductivecore, typically a metal core as discussed below; (II) applyingelectrophoretically any of the previously discussed electrodepositablecoating compositions onto all exposed surfaces of the core to form aconformal dielectric coating thereon; (III) ablating a surface of theconformal dielectric coating in a predetermined pattern to expose asection of the core; (IV) applying a layer of metal for example, copper,to all surfaces to form metallized vias in the core; and (V) applying aresinous photosensitive layer to the metal layer.

The substrate or core can comprise any of a variety of electroconductivesubstrates, particularly metal substrates, for example, untreated orgalvanized steel, aluminum, gold, nickel, copper, magnesium or alloys ofany of the foregoing metals, as well as conductive carbon coatedmaterials. Also, the core has two major surfaces and edges and can havea thickness ranging from 10 to 100 microns, typically from 25 to 100microns.

It should be understood that for purposes of the processes of thepresent invention the formation of metallized vias “in the core” isintended to encompass the formation of “through vias” (i.e., theformation of metallized holes extending through the core from one majorsurface to the other) to provide through connections, as well as theformation of “blind vias” (i.e., the formation of metallized holesextending through the dielectric coating only to, but not through, thecore) to provide connections, for example, to ground or power. Also, forpurposes of the present invention, the formation of metallized viasextending “through the core” is intended to encompass the formation ofthrough vias only. Likewise, the formation of metallized vias extending“to the core” is intended to encompass the formation of blind vias only.

In a particular embodiment of the present invention, the core is a metalsubstrate selected from perforate copper foil, an iron-nickel alloy orcombinations thereof. In one embodiment of the present invention, thecore comprises an iron-nickel alloy commercially available as INVAR(trademark of Imphy S. A., 168 Rue de Rivoli, Paris, France) comprisingapproximately 64 weight percent iron and 36 weight percent nickel. Thisalloy has a low coefficient of thermal expansion comparable to that ofthe silicon materials typically used to prepare chips. This property isdesirable in order to prevent failure of adhesive joints betweensuccessively larger or smaller scale layers of a chip scale package dueto thermal cycling during normal use. When an iron-nickel alloy is usedas the metal core, a layer of metal, usually copper, typically isapplied to all surfaces of the iron-nickel alloy core to ensure optimalconductivity. This layer of metal as well as that applied in step (IV)can be applied by conventional means and for example, by electroplatingand metal vapor deposition techniques, electroless plating, andtypically has a thickness of from 1 to 10 microns.

By “perforate metal core” is meant a mesh sheet having a plurality ofholes or vias spaced at regular intervals. The diameter of the holesusually is about 200 microns, but may be larger or smaller as necessary,provided that the diameter is large enough to accommodate all the layersapplied in the process of the present invention without the holesbecoming obstructed. The center-to-center spacing of the holes typicallyis about 500 microns, but, likewise, may be larger or smaller asnecessary. Via density can range from 500 to 10,000 holes per squareinch (77.5 to 1,550 holes per square centimeter).

Any of the electrodepositable coating compositions described in detailbelow can be electrophoretically applied to the electroconductive core.The applied voltage for electrodeposition may be varied and can be, forexample, as low as 1 volt to as high as several thousand volts, buttypically between 50 and 500 volts. The current density is usuallybetween 0.5 ampere and 5 amperes per square foot (0.5 to 5 milliamperesper square centimeter) and tends to decrease during electrodepositionindicating the formation of an insulating conformal film on all exposedsurfaces of the core. As used herein, in the specification and in theclaims, by “conformal” film or coating is meant a film or coating havinga substantially uniform thickness which conforms to the substratetopography, including the surfaces within (but, preferably, notoccluding) the holes. After the coating has been applied byelectrodeposition, it typically is thermally cured at elevatedtemperatures ranging from 90° to 300° C. for a period of 1 to 40 minutesto form a conformal dielectric coating over all exposed surfaces of thecore.

It should be understood, that for purposes of the processes of thepresent invention, any of the electrodepositable coating compositions(described in detail below) can be applied by a variety of applicationtechniques other than electrodeposition which are well-know in the art,for example, by roll-coating or spray application techniques. In suchinstances, it may be desirable to prepare the composition at higherresin solids content. Also, for such applications, the resinous bindermay or may not include solubilizing or neutralizing acids and amines toform cationic and anionic salt groups, respectively.

The dielectric coating is of uniform thickness and often can be no morethan 50 microns, usually no more than 25 microns, and typically no morethan 20 microns. A lower film thickness is desirable for a variety ofreasons. For example, a dielectric coating having a low film thicknessallows for smaller scale circuitry. Also, a coating have a lowdielectric constant (as discussed above) allows for a dielectric coatinghaving lower film thickness and also minimizes capacitive couplingbetween adjacent signal traces.

Those skilled in the art would recognize that prior to theelectrophoretic application of the dielectric coating, the core surfacemay be pretreated or otherwise prepared for the application of thedielectric. For example, cleaning, rinsing, and/or treatment with anadhesion promoter prior to application of the dielectric may beappropriate.

After application of the dielectric coating, the surface of thedielectric coating is ablated in a predetermined pattern to expose oneor more sections of the core. Such ablation typically is performed usinga laser or by other conventional techniques, for example, mechanicaldrilling and chemical or plasma etching techniques.

Metallization is performed after the ablation step by applying a layerof metal to all surfaces, allowing for the formation of metallized viasin the core. Suitable metals include copper or any metal or alloy withsufficient conductive properties. The metal is typically applied byelectroplating or any other suitable method known in the art to providea uniform metal layer. The thickness of this metal layer can range from1 to 50 microns, typically from 5 to 25 microns.

To enhance the adhesion of the metal layer to the dielectric polymer,prior to the metallization step all surfaces can be treated with ionbeam, electron beam, corona discharge or plasma bombardment followed byapplication of an adhesion promoter layer to all surfaces. The adhesionpromoter layer can range from 50 to 5000 Ångstroms thick and typicallyis a metal or metal oxide selected from chromium, titanium, nickel,cobalt, cesium, iron, aluminum, copper, gold, tungsten and zinc, andalloys and oxides thereof.

After metallization, a resinous photosensitive layer (i.e. “photoresist”or “resist”) can be applied to the metal layer. Optionally, prior toapplication of the photoresist, the metallized substrate can be cleanedand/or pretreated; e.g., treated with an acid etchant to remove oxidizedmetal. The resinous photosensitive layer can be a positive or negativephotoresist. The photoresist layer can have a thickness ranging from 1to 50 microns, typically from 5 to 25 microns, and can be applied by anymethod known to those skilled in the photolithographic processing art.Additive or subtractive processing methods may be used to create thedesired circuit patterns.

Suitable positive-acting photosensitive resins include any of thoseknown to practitioners skilled in the art. Examples includedinitrobenzyl functional polymers such as those disclosed in U.S. Pat.No. 5,600,035, columns 3–15. Such resins have a high degree ofphotosensitivity. In one embodiment, the resinous photosensitive layeris a composition comprising a dinitrobenzyl functional polymer,typically applied by spraying.

In a separate embodiment, the resinous photosensitive layer comprises anelectrodepositable composition comprising a dinitrobenzyl functionalpolyurethane and an epoxy-amine polymer such as that described inExamples 3–6 of U.S. Pat. No. 5,600,035.

Negative-acting photoresists include liquid or dry-film typecompositions. Any of the previously described liquid compositions may beapplied by spray; roll-coating; spin-coating, curtain-coating,screen-coating, immersion coating, or electrodeposition applicationtechniques.

In one embodiment, the liquid photoresists are applied byelectrodeposition, usually by cationic electrodeposition.Electrodepositable photoresist compositions comprise an ionic, polymericmaterial which may be cationic or anionic, and may be selected frompolyesters, polyurethanes, acrylics, and polyepoxides. Examples ofphotoresists applied by anionic electrodeposition are shown in U.S. Pat.No. 3,738,835. Photoresists applied by cationic electrodeposition aredescribed in U.S. Pat. No. 4,592,816. Examples of dry-film photoresistsinclude those disclosed in U.S. Pat. Nos. 3,469,982, 4,378,264, and4,343,885. Dry-film photoresists are typically laminated onto thesurface such as by application of hot rollers.

Note that after application of the photosensitive layer, the multi-layersubstrate may be packaged at this point allowing for transport andprocessing of any subsequent steps at a remote location.

In an embodiment of the invention, after the photosensitive layer isapplied, a photo-mask having a desired pattern may be placed over thephotosensitive layer and the layered substrate exposed to a sufficientlevel of a suitable radiation source, typically an actinic radiationsource. As used herein, the term “sufficient level of radiation” refersto that level of radiation which polymerizes the monomers in theradiation-exposed areas in the case of negative acting resists, or whichde-polymerizes the polymer or renders the polymer more soluble in thecase of positive acting resists. This results in a solubilitydifferential between the radiation-exposed and radiation-shielded areas.

The photo-mask may be removed after exposure to the radiation source andthe layered substrate developed using conventional developing solutionsto remove more soluble portions of the photosensitive layer, and uncoverselected areas of the underlying metal layer. The metal uncovered maythen be etched using metal etchants which convert the metal to watersoluble metal complexes. The soluble complexes may be removed by waterspraying.

The photosensitive layer protects the underlying substrate during theetching step. The remaining photosensitive layer, which is impervious tothe etchants, may then be removed by a chemical stripping process toprovide a circuit pattern connected by the metallized vias.

After preparation of the circuit pattern on the multi-layered substrate,other circuit components may be attached to form a circuit assembly.Additional components include, for example, one or more smaller scalecomponents such as semiconductor chips, interposer layers, larger scalecircuit cards or mother boards and active or passive components. Notethat interposers used in the preparation of the circuit assembly may beprepared using appropriate steps of the process of the presentinvention. Components may be attached using conventional adhesives,surface mount techniques, wire bonding or flip chip techniques. High viadensity in the multi-layer circuit assembly prepared in accordance withthe present invention allows for more electrical interconnects fromhighly functional chips to the packages in the assembly.

In another embodiment, the present invention is directed to a processfor fabricating a circuit assembly comprising the steps of (I) providinga core, such as any of the previously described metal cores; (II)providing a photoresist, such as any of the photoresist compositionsdescribed above, at predetermined locations on the surface of the core;(III) applying electrophoretically any of the electrodepositable coatingcompositions described in detail below over the core of step (II),wherein the coating composition is deposited electrophoretically overall surfaces of the core except at the locations having the photoresistthereon; (IV) curing the electrophoretically applied coating compositionunder the curing conditions described above to form a cured conformaldielectric layer over all surfaces of the core except at the locationshaving the photoresist thereon; (V) removing the photoresist, asdescribed above, to form a circuit assembly having vias extending to themetal core at the locations previously covered with the resist; and (VI)optionally, applying a layer of metal, usually copper metal, to allsurfaces of the circuit assembly of step (V) by any of the previouslydescribed methods for metallizing to form metallized vias extending tothe core. In a particular embodiment of the present invention, prior toproviding the resist in step (II) at predetermined locations on thesurface of the metal core, a layer of metal, typically copper metal, isapplied to the metal core.

The electrodepositable coating compositions suitable for use in any ofthe previously discussed processes comprise a resinous phase dispersedin an aqueous medium. The resinous phase comprises: (a) an ungelled,active hydrogen-containing, ionic salt group-containing resin; and (b) acuring agent reactive with the active hydrogens of the resin (a).

In one embodiment of the present invention, the resinous phase has acovalently bonded halogen content based on total weight of resin solidspresent in said resinous phase such that when the composition iselectrodeposited and cured to form a cured film, the cured film passesflame resistance testing in accordance with IPC-TM-650, and has adielectric constant of less than or equal to 3.50. It should beunderstood that for purposes of the present invention, by “covalentlybonded halogen” is meant a halogen atom that is covalently bonded asopposed to a halogen ion, for example, a chloride ion in aqueoussolution.

The resinous phase of the electrodepositable coating composition of thepresent invention can have a covalently bonded halogen content of atleast 1 weight percent, usually at least 2 weight percent, often atleast 5 weight percent, and typically at least 10 weight percent. Also,the resinous phase of the electrodepositable coating composition of thepresent invention can have a covalently bonded halogen content of lessthan 50 weight percent, usually less than 30 weight percent, often lessthan 25 weight percent, and typically less than 20 weight percent. Theresinous phase of the electrodepositable coating composition can have acovalently bonded halogen content which can range between anycombination of these values, inclusive of the recited values, providedthat the covalently bonded halogen content is sufficient to provide acured coating which passes flame resistance testing in accordance withIPC-TM-650 as described below.

Additionally, it should be noted that the covalently bonded halogencontent of the resinous phase can be derived from halogen atomscovalently bonded to one or both of the resin (a) and the curing agent(b), or halogen atoms covalently bonded to a compound (c) which isdifferent from and present as a component in the resinous phase of theelectrodepositable coating composition in addition to the resin (a) andthe curing agent (b).

As discussed above, for purposes of the present invention, flameresistance is tested in accordance with IPC-TM-650, Test Methods Manual,Number 2.3.10, “Flammability of Laminate”, Revision B, available fromthe Institute of Interconnecting and Packaging Electronic Circuits, 2215Sanders Road, Northbrook, Ill.

When the electrodepositable coating composition described above iselectrophoretically deposited and cured to form a cured film (asdescribed in detail below), the cured film can have a dielectricconstant of no more than 3.50, often no more than 3.30, usually of nomore than 3.00, and typically no more than 2.80. Also, the cured filmtypically has a dielectric loss factor of less than or equal to 0.02,usually less than or equal to 0.15, and can be less than or equal to0.01.

A dielectric material is a non-conducting substance or insulator. The“dielectric constant” is an index or measure of the ability of adielectric material to store an electric charge. The dielectric constantis directly proportional to the capacitance of a material, which meansthat the capacitance is reduced if the dielectric constant of a materialis reduced. A low dielectric material is desired for high frequency,high speed digital where the capacitances of substrates and coatings arecritical to the reliable functioning of circuits. For example, presentcomputer operations are limited by coupling capacitance between circuitpaths and integrated circuits on multi-layer assemblies since computingspeed between integrated circuits is reduced by this capacitance and thepower required to operate is increased. See Thompson, Larry F., et al.,Polymers for Microelectronics, presented at the 203^(rd) NationalMeeting of American Chemical Society, Apr. 5–10, 1992.

The “dielectric loss factor” is the power dissipated by a dielectricmaterial as the friction of its molecules opposes the molecular motionproduced by an alternating electric field. See I. Gilleo, Ken, Handbookof Flexible Circuits, at p. 242, Van Nostrand Reinhold, New York (1991).See also, James J. Licari and Laura A. Hughes, Handbook of PolymerCoatings for Electronics, pp. 114–18, 2^(nd) ed., Noyes Publication(1990) for a detailed discussion of dielectric materials and dielectricconstant.

For purposes of the present invention, the dielectric constant of thecured electrodepositable coating composition is determined at afrequency of 1 megahertz using electrochemical impedance spectroscopy asfollows.

The coating sample is prepared by application of the electrodepositablecomposition to a steel substrate and subsequent curing to provide acured dielectric coating having a film thickness of 0.85 mil (20.83microns). A 32 square centimeter free film of the cured dielectriccoating is placed in the electrochemical cell with 150 milliliters ofelectrolyte solution (1 M NaCl) and allowed to equilibrate for one hour.An AC potential of 100 mV is applied to the sample and the impedance ismeasured from 1.5 megahertz to 1 hertz frequency range. The methodemploys a platinum-on-niobium expanded mesh counter electrode and asingle junction silver/silver chloride reference electrode. Thedielectric constant of the cured coating is determined by calculatingthe capacitance at 1 megahertz, 1 kilohertz, and 63 hertz, and solvingthe following equation for E.C=E _(o) EA/dwhere C is the measured capacitance at discrete frequency (in Farads);E_(o) is the permitivity of free space (8.854187817¹²); A is the samplearea (32 square centimeters; d is the coating thickness; and E is thedielectric constant. It should be noted the values for dielectricconstant as used in the specification and in the claims is thedielectric constant determined as described above at a frequency of 1megahertz. Likewise, values for the dielectric loss factor as used inthe specification and in the claims is the difference between thedielectric constant measured at a frequency of 1 megahertz as describedabove, and the dielectric constant for the same material measured at afrequency of 1.1 megahertz.

The electrodepositable coating compositions useful in the processes ofthe present invention comprise as a main film-former, an ungelled,active hydrogen ionic group-containing electrodepositable resin (a). Awide variety of electrodepositable film-forming polymers are known andcan be used in the electrodepositable coating compositions of thepresent invention so long as the polymers are “water dispersible,” i.e.,adapted to be solubilized, dispersed or emulsified in water. The waterdispersible polymer is ionic in nature, that is, the polymer can containanionic functional groups to impart a negative charge or cationicfunctional groups to impart a positive charge. In a particularembodiment of the present invention, the resin (a) comprises cationicsalt groups, usually cationic amine salt groups.

By “ungelled” is meant the resins are substantially free of crosslinkingand have an intrinsic viscosity when dissolved in a suitable solvent, asdetermined, for example, in accordance with ASTM-D1795 or ASTM-D4243.The intrinsic viscosity of the reaction product is an indication of itsmolecular weight. A gelled reaction product, on the other hand, since itis of essentially infinitely high molecular weight, will have anintrinsic viscosity too high to measure. As used herein, a reactionproduct that is “substantially free of crosslinking” refers to areaction product that has a weight average molecular weight (Mw), asdetermined by gel permeation chromatography, of less than 1,000,000.

Also, as used herein, the term “polymer” is meant to refer to oligomersand both homopolymers and copolymers. Unless stated otherwise, molecularweights are number average molecular weights for polymeric materialsindicated as “Mn” and obtained by gel permeation chromatography using apolystyrene standard in an art-recognized manner.

Non-limiting examples of film-forming resins suitable for use as theresin (a) in anionic electrodepositable coating compositions includebase-solubilized, carboxylic acid group-containing polymers such as thereaction product or adduct of a drying oil or semi-drying fatty acidester with a dicarboxylic acid or anhydride; and the reaction product ofa fatty acid ester, unsaturated acid or anhydride and any additionalunsaturated modifying materials which are further reacted with polyol.Also suitable are the at least partially neutralized interpolymers ofhydroxy-alkyl esters of unsaturated carboxylic acids, unsaturatedcarboxylic acid and at least one other ethylenically unsaturatedmonomer. Still another suitable electrodepositable resin comprises analkyd-aminoplast vehicle, i.e., a vehicle containing an alkyd resin andan amine-aldehyde resin. Another suitable anionic electrodepositableresin composition comprises mixed esters of a resinous polyol. Thesecompositions are described in detail in U.S. Pat. No. 3,749,657 at col.9, lines 1 to 75 and col. 10, lines 1 to 13, all of which are hereinincorporated by reference. Other acid functional polymers can also beused such as phosphatized polyepoxide or phosphatized acrylic polymersas are well known to those skilled in the art. Additionally, suitablefor use as the resin (a) are those resins comprising one or more pendentcarbamate functional groups, for example, those described in U.S. Pat.No. 6,165,338.

In one particular embodiment of the present invention, the activehydrogen-containing ionic electrodepositable resin (a) is cationic andcapable of deposition on a cathode. Non-limiting examples of suchcationic film-forming resins include amine salt group-containing resinssuch as the acid-solubilized reaction products of polyepoxides andprimary or secondary amines such as those described in U.S. Pat. Nos.3,663,389; 3,984,299; 3,947,338; and 3,947,339. Usually, these aminesalt group-containing resins are used in combination with a blockedisocyanate curing agent as described in detail below. The isocyanate canbe fully blocked as described in the aforementioned U.S. Pat. No.3,984,299 or the isocyanate can be partially blocked and reacted withthe resin backbone such as described in U.S. Pat. No. 3,947,338. Also,one-component compositions as described in U.S. Pat. No. 4,134,866 andDE-OS No. 2,707,405 can be used in the electrodepositable coatingcompositions of the present invention as the resin (a). Besides theepoxy-amine reaction products discussed immediately above, the resin (a)can also be selected from cationic acrylic resins such as thosedescribed in U.S. Pat. Nos. 3,455,806 and 3,928,157.

Besides amine salt group-containing resins, quaternary ammonium saltgroup-containing resins can also be employed. Examples of these resinsinclude those which are formed from reacting an organic polyepoxide witha tertiary amine salt. Such resins are described in U.S. Pat. Nos.3,962,165; 3,975,346; and 4,001,101. Examples of other cationic resinsare ternary sulfonium salt group-containing resins and quaternaryphosphonium salt-group containing resins such as those described in U.S.Pat. Nos. 3,793,278 and 3,984,922, respectively. Also, film-formingresins which cure via transesterification such as described in EuropeanApplication No. 12463 can be used. Further, cationic compositionsprepared from Mannich bases such as described in U.S. Pat. No. 4,134,932can be used.

In one embodiment of the present invention, the resin (a) can compriseone or more positively charged resins which contain primary and/orsecondary amine groups. Such resins are described in U.S. Pat. Nos.3,663,389; 3,947,339; and 4,116,900. In U.S. Pat. No. 3,947,339, apolyketimine derivative of a polyamine such as diethylenetriamine ortriethylenetetraamine is reacted with a polyepoxide. When the reactionproduct is neutralized with acid and dispersed in water, free primaryamine groups are generated. Also, equivalent products are formed whenpolyepoxide is reacted with excess polyamines such as diethylenetriamineand triethylenetetraamine and the excess polyamine vacuum stripped fromthe reaction mixture. Such products are described in U.S. Pat. Nos.3,663,389 and 4,116,900.

Mixtures of the above-described ionic resins also can be usedadvantageously. In one embodiment of the present invention, the resin(a) comprises a polymer having cationic salt groups and is selected froma polyepoxide-based polymer having primary, secondary and/or tertiaryamine groups (such as those described above) and an acrylic polymerhaving hydroxyl and/or amine functional groups.

As previously discussed, in one particular embodiment of the presentinvention, the resin (a) comprises cationic salt groups. In thisinstance, such cationic salt groups typically are formed by solubilizingthe resin with an inorganic or organic acid such as those conventionallyused in electrodepositable compositions. Suitable examples ofsolubilizing acids include, but are not limited to, sulfamic, acetic,lactic, and formic acids. Sulfamic and lactic acids are most commonlyemployed.

Also, as aforementioned, the covalently bonded halogen content of theresinous phase of the electrodepositable coating composition can bederived from halogen atoms covalently bonded to the resin (a). In suchinstances, the covalently bonded halogen content can be attributed to areactant used to form any of the film-forming ionic resins describedabove. For example, in the case of an anionic group-containing polymer,the resin may be the reaction product of a halogenated phenol, forexample a halogenated polyhydric phenol such as chlorinated orbrominated bisphenol A with an epoxy compound followed by reaction withphosphoric acid, or alternatively, an epoxy compound reacted with ahalogenated carboxylic acid, followed by reaction of any residual epoxygroups with phosphoric acid. The acid groups can then be solubilizedwith an amine. Likewise, in the case of a cationic salt group-containingpolymer, the resin may be the reaction product of diglycidyl ether ofBisphenol A with a halogenated phenol, follow by reaction of anyresidual epoxy groups with an amine. The reaction product then can besolubilized with an acid.

In one embodiment of the present invention, the covalently bondedhalogen content of the resin (a) can be derived from a halogenatedphenol, trichlorobutylene oxide, and mixtures thereof. In anotherembodiment of the present invention, the covalently bonded halogencontent of the resin (a) is derived from a halogenated polyhydricphenol, for example, chlorinated bisphenol A such astetrachlorobisphenol A, or a brominated bisphenol A such astetrabromobisphenol A. In a further embodiment of the present invention,the covalently bonded halogen content of the resin (a) can be derivedfrom a halogenated epoxy compound, for example the diglycidyl ether of ahalogenated bisphenol A.

The active hydrogen-containing ionic electrodepositable resin (a)described above can be present in the electrodepositable coatingcomposition of the present invention in amounts ranging from 5 to 90percent by weight, usually 10 to 80 percent by weight, often 10 to 70percent by weight, and typically 10 to 30 percent by weight based ontotal weight of the electrodepositable coating composition.

As mentioned above, the resinous phase of the electrodepositable coatingcomposition of the present invention further comprises (b) a curingagent adapted to react with the active hydrogens of the ionicelectrodepositable resin (a) described immediately above. Both blockedorganic polyisocyanate and aminoplast curing agents are suitable for usein the present invention, although blocked isocyanates typically areemployed for cathodic electrodeposition.

Aminoplast resins, which are common curing agents for anionicelectrodeposition, are the condensation products of amines or amideswith aldehydes. Examples of suitable amine or amides are melamine,benzoguanamine, urea and similar compounds. Generally, the aldehydeemployed is formaldehyde, although products can be made from otheraldehydes such as acetaldehyde and furfural. The condensation productscontain methylol groups or similar alkylol groups depending on theparticular aldehyde employed. Preferably, these methylol groups areetherified by reaction with an alcohol. Various alcohols employedinclude monohydric alcohols containing from 1 to 4 carbon atoms such asmethanol, ethanol, isopropanol, and n-butanol, with methanol beingpreferred. Aminoplast resins are commercially available from AmericanCyanamid Co. under the trademark CYMEL and from Monsanto Chemical Co.under the trademark RESIMENE.

The aminoplast curing agents typically are utilized in conjunction withthe active hydrogen containing anionic electrodepositable resin inamounts ranging from 1 to 90, often from 5 percent to about 60 percentby weight, preferably from 20 percent to 40 percent by weight, thepercentages based on the total weight of the resin solids in theelectrodepositable coating composition.

The curing agents commonly employed in cathodic electrodepositioncompositions are blocked polyisocyanates. The polyisocyanates can befully blocked as described in U.S. Pat. No. 3,984,299 column 1 lines 1to 68, column 2 and column 3 lines 1 to 15, or partially blocked andreacted with the polymer backbone as described in U.S. Pat. No.3,947,338 column 2 lines 65 to 68, column 3 and column 4 lines 1 to 30,which are incorporated by reference herein. By “blocked” is meant thatthe isocyanate groups have been reacted with a compound such that theresultant blocked isocyanate group is stable to active hydrogens atambient temperature but reactive with active hydrogens in the filmforming polymer at elevated temperatures usually between 90° C. and 200°C.

Suitable polyisocyanates include aromatic and aliphatic polyisocyanates,including cycloaliphatic polyisocyanates and representative examplesinclude diphenylmethane-4,4′-diisocyanate (MDI), 2,4- or 2,6-toluenediisocyanate (TDI), including mixtures thereof, p-phenylenediisocyanate, tetramethylene and hexamethylene diisocyanates,dicyclohexylmethane-4,4′-diisocyanate, isophorone diisocyanate, mixturesof phenylmethane-4,4′-diisocyanate and polymethylenepolyphenylisocyanate. Higher polyisocyanates such as triisocyanates canbe used. An example would includetriphenylmethane-4,4′,4″-triisocyanate. Isocyanate prepolymers withpolyols such as neopentyl glycol and trimethylolpropane and withpolymeric polyols such as polycaprolactone diols and triols (NCO/OHequivalent ratio greater than 1) can also be used.

The polyisocyanate curing agents typically are utilized in conjunctionwith the active hydrogen containing cationic electrodepositable resin(a) in amounts ranging from ranging from 1 to 90 percent by weight,usually 1 to 80 percent by weight, often 1 to 70 percent by weight, andtypically 1 to 15 percent by weight based on total weight of theelectrodepositable coating composition.

Also suitable are beta-hydroxy urethane curing agents such as thosedescribed in U.S. Pat. Nos. 4,435,559 and 5,250,164. Such beta-hydroxyurethanes are formed from an isocyanate compound, for example, any ofthose described immediately above, a 1,2-polyol and/or a conventionalblocking such as monoalcohol. Also suitable are the secondary amineblocked aliphatic and cycloaliphatic isocyanates described in U.S. Pat.Nos. 4,495,229 and 5,188,716.

As previously discussed, in one embodiment of the present invention, thecuring agent (b) can have a covalently bonded halogen content of up to60 weight percent, often from 1 to 50 weight percent, often from 2 to 80weight percent, usually from 5 to 25 weight percent, and can be from 10to 20 weigh percent based on weight of total resin solids present in thecuring agent (b). In such instances, the covalently bonded halogencontent present in the curing agent (b) can be derived from for example,a halogen containing blocked isocyanate can be prepared by at leastpartially blocking 4-chloro-6-methyl-1,3-phenylene diisocyanate with asuitable blocking agent such as 2-butoxy ethanol, If partially blocked,any residual isocyanate groups can be reacted with a polyol such astrimethol propane thereby increasing the molecular weight of the curingagent.

In a further embodiment of the present invention, the covalently bondedhalogen content present in the resinous phase of the electrodepositablecoating composition can be derived from a component (c) which isdifferent from and present in addition to the resin (a) and the curingagent (b). In such instances, the component (c) typically is acovalently bonded halogen-containing compound selected from the groupconsisting of halogenated polyolefin, halogenated phosphate ester,halogenated phenol such as any of the halogenated phenols describedabove.

As aforementioned, the covalently bonded halogen content present in theresinous phase of the electrodepositable coating composition can bederived from the resin (a), the curing agent (b), the component (c), orany combination of the foregoing, provided that the covalently bondedhalogen content is sufficient to ensure that the resultantelectrodeposition coating when electrophoretically applied and curedpasses flame resistance testing in accordance with IPC-TM-650 aspreviously discussed. The covalently bonded halogen content of theresinous phase of the electrodepositable coating composition also shouldbe present in an amount insufficient to adversely affect theelectrodeposition process and/or the resulting dielectric coatingproperties.

In one embodiment of the present invention, the electrodepositablecoating compostion can further comprise a rheology modifier which canassist in the deposition of a smooth and uniform thickness of thedielectric coating on the surface of the hole or via walls as well asthe edges at the via openings. Any of a variety of the rheologymodifiers well-known in the coatings art can be employed for thispurpose.

One suitable rheology modifier comprises a cationic microgel dispersionprepared by dispersing in aqueous medium a mixture of a cationicpolyepoxide-amine reaction product which contains amine groups,typically primary amine groups, secondary amine groups and mixturesthereof, and a polyepoxide crosslinking agent, and heating the mixtureto a temperature sufficient to crosslink the mixture, thus forming acationic microgel dispersion. Such cationic microgel dispersions andtheir preparation are described in detail in U.S. Pat. No. 5,096,556 atcolumn 1, line 66 to column 5, line 13, incorporated by referenceherein. Other suitable rheology modifiers include the cationic microgeldispersion having a shell-core morphology described in detail in EP 0272 500 B1. This microgel is prepared by emulsification in aqueousmedium of a cationic film-forming resin and a thermosetting crosslinkingagent, and heating the resultant emulsion to a temperature sufficient tocrosslink the two components.

The cationic microgel is present in the electrodepositable coatingcomposition in an amount sufficient to effect adequate rheology controland hole edge coverage, but insufficient to adversely affect flow of theelectrodepositable composition upon application or surface roughness ofthe cured coating. For example, the cationic microgels describedimmediately above can be present in the resinous phase of theelectrodepositable coating composition in an amount ranging from 0.1 to30 weight percent, typically from 1 to 10 weight percent based on weightof total resin solids present in the resinous phase.

The electrodepositable coating composition is in the form of an aqueousdispersion. The term “dispersion” is believed to be a two-phasetransparent, translucent or opaque resinous system in which the resin isin the dispersed phase and the water is in the continuous phase. Theaverage particle size of the resinous phase is generally less than 1.0micron, usually less than 0.5 micron, and typically less than 0.15micron.

The concentration of the resinous phase in the aqueous medium is atleast 1, and usually from 2 to 60 percent by weight based on totalweight of the aqueous dispersion. When the compositions of the presentinvention are in the form of resin concentrates, they generally have aresin solids content of 20 to 60 percent by weight based on weight ofthe aqueous dispersion.

Electrodepositable coating compositions of the invention typically aresupplied as two components: (1) a clear resin feed, which includes,generally, the active hydrogen-containing ionic electrodepositableresin, i.e., the main film-forming polymer, the curing agent, and anyadditional water-dispersible, non-pigmented components; and (2) apigment paste, which, generally, includes one or more pigments, awater-dispersible grind resin which can be the same or different fromthe main-film forming polymer, and, optionally, additives such ascatalysts, and wetting or dispersing aids. Electrodepositable coatingcomponents (1) and (2) are dispersed in an aqueous medium whichcomprises water and, usually, coalescing solvents to form anelectrodeposition bath. Alternatively, the electrodepositablecomposition of the present invention can be supplied as a one-componentcomposition. In a particular embodiment of the present invention, theelectrodepositable coating composition can be supplied as asubstantially pigment-free, one-component composition.

It should be appreciated that there are various methods by which thecomponent (c), when employed, can be incorporated into theelectrodepositable coating composition in the form of anelectrodeposition bath. The component (c) can be incorporated “neat”,that is, the component (c) or an aqueous solution thereof can be addeddirectly to the dispersed electrodeposition composition components (1)and (2), or if applicable, to the dispersed one-componentelectrodeposition composition. Alternatively, the component (c) can beadmixed with or dispersed in the clear resin feed (or any of theindividual clear resin feed components, for example the film-formingresin or the curing agent) prior to dispersing components (1) and (2) inthe aqueous medium. Further, the component (c) can be admixed with ordispersed in the pigment paste, or any of the individual pigment pastecomponents, for example, the pigment grind resin prior to dispersingcomponents (1) and (2) in the aqueous medium. Finally the component (c)can be added on-line directly to the electrodeposition bath.

The electrodepositable coating composition in the form of anelectrodeposition bath typically has a resin solids content within therange of 5 to 25 percent by weight based on total weight of theelectrodeposition bath.

As aforementioned, besides water, the aqueous medium may contain acoalescing solvent. Useful coalescing solvents include hydrocarbons,alcohols, esters, ethers and ketones. Usual coalescing solvents includealcohols, polyols and ketones. Specific coalescing solvents includeisopropanol, butanol, 2-ethylhexanol, isophorone, 2-methoxypentanone,ethylene and propylene glycol and glycol ethers such as monoethyl,monobutyl and monohexyl ethers of ethylene glycol. The amount ofcoalescing solvent is generally between about 0.01 and 25 percent andwhen used, preferably from about 0.05 to about 5 percent by weight basedon total weight of the aqueous medium.

Although typically substantially free of pigment, if desired, a pigmentcomposition and/or various additives such as surfactants, wetting agentsor catalyst can be included in the dispersion. The pigment compositionmay be of the conventional type comprising pigments, for example, ironoxides, strontium chromate, carbon black, titanium dioxide, talc, bariumsulfate, as well as color-imparting pigments well known in the art. Theelectrodeposition bath can be, if desired, essentially free of chrome-and/or lead-containing pigments.

The pigment content of the dispersion is usually expressed as apigment-to-resin ratio. In the practice of the invention, when pigmentis employed, the pigment-to-resin ratio is usually within the range ofabout 0.02 to 1:1. The other additives mentioned above are usually inthe dispersion in amounts ranging from 0.01 to 10 percent by weightbased on weight of resin solids.

Illustrating the invention are the following examples which are not tobe considered as limiting the invention to their details. Unlessotherwise indicated, all parts and percentages in the followingexamples, as well as throughout the specification, are by weight.

EXAMPLES

The following describes the preparation of a circuitized substrate usingthe process of the present invention. Example A describes thepreparation of a resinous binder comprised of a tetrabromobisphenol Awhich is used in the electrodepositable coating composition ofExample 1. The electrodepositable coating composition of Example 1 inthe form of an electrodeposition bath was used to provide a conformaldielectric coating on a perforate substrate, which was subsequentlymetallized, photoimaged, developed and stripped as described below.

Example A

This example describes the preparation of a cationic resinous binderused in the electrodepositable coating composition of the followingExample 1. The resinous binder was prepared as described below from thefollowing ingredients. All values listed represent parts by weight ingrams.

Ingredients Example A Crosslinker¹ 1882 Diethylene glycol monobutyl108.78 ether formal EPON ® 828² 755.30 Tetrabromobisphenol A 694.90TETRONIC 150R1³ 0.33 Diethanolamine 51.55 Aminopropyl diethanolamine113.2 Distillate removed (67.66) Sulfamic acid 45.17 Deionized water2714 Lactic acid⁴ 1.70 Resin intermediate⁵ 244.7 Gum rosin⁵ 27.49Deionized water 2875 ¹Polyisocyanate curing agent prepared from thefollowing ingredients: Parts by Weight Ingredients (grams) Ethanol 92.0Propylene glycol 456.0 Polyol^(a) 739.5 Methylisobutyl ketone 476.5Diethylene glycol monobutyl 92.8 ether formal^(b) DESMODUR LS2096^(c)1320.0 Methylisobutyl ketone 76.50 ^(a)Bisphenol A/ethylene oxide adductavailable from BASF Corporation as MACOL 98B. ^(b)Available from BASFCorporation as MAZON 1651. ^(c)Isocyanate available from BayerCorporation.  The first five ingredients were charged to a suitablyequipped reaction vessel under agitation. As the temperature reachedabout 25° C., the addition of DESMODUR LS2096 was begun. Temperature wasincreased to 105° C. at which time the last addition of methylisobutylketone was made. Temperature was held at 100° C. as the reaction wasmonitored for the disappearance of NCO by infrared spectroscopy at whichtime, the temperature was reduced to 80° C. ²Diglycidyl ether ofbisphenol A available from Shell Oil and Chemical Company. ³Surfactantavailable from BASF Corporation. ⁴88% aqueous solution. ⁵Cationic resinprepared from the following ingredients. Parts by Weight Ingredients(grams) MAZEEN 355 70^(a) 603.34 Acetic acid 5.99 Dibutyltindilaurate0.66 Toluene diisocyanate 87.17 Sulfamic acid 38.79 Deionized water1289.89 ^(a)Aminediol available from BASF Corporation  The first twoingredients were charged to a suitably equipped reaction vessel andagitated for 10 minutes at which time dibutyl- tindilaurate was added.The toluene diisocyanate was added slowly as the reaction was permittedto exotherm to a temperature of 100° C. and held at that temperatureuntil the disappearance of all NCO as monitored by infraredspectroscopy. The resin thus prepared was solubilized with the additionof sulfamic acid and de- ionized water under agitation. The finaldispersion had a measured resin solids content of 26 percent by weight.

The crosslinker was added to a suitably equipped reaction vessel. Thenext four ingredients were added to the reaction vessel under mildagitation and the reaction mixture was heated to a temperature of 75° C.at which time the diethanolamine was added. The reaction mixture washeld at that temperature for a period of 30 minutes. The aminopropyldiethanolamine was then added and the reaction mixture was permitted toexotherm to 132° C. and held at that temperature for a period of 2hours. Distillate was removed. For solubilization, the reaction productwas added under mild agitation to an admixture of the sulfamic acid,deionized water, lactic acid solution and cationic resin intermediate.The gum rosin solution was then added to the solubized resin, followedby deionized water in two sequential additions. Excess water and solventwere removed by stripping under vacuum at a temperature of 60°–65° C.The final reaction product had a measured resin solids content ofapproximately 40 percent by weight.

Example 1

The following example describes the preparation of an electrodepositablecoating composition of the present invention in the form of anelectrodeposition bath comprising the cationic resinous binder ofExample A above. The electrodepositable coating composition was preparedas described below from the following ingredients. All values listedrepresent parts by weight in grams.

Ingredients Example 1 Resinous binder of Example A 704.9 Hexylcellosolve 28.5 E6278¹ 13.2 Deionized water 3053.4 ¹Catalyst paste,available from PPG Industries, Inc.

The ingredients listed above were combined and mixed with mildagitation. The composition was ultrafiltered 50% and reconstituted withdeionized water.

Preparation of a Circuitized Substrate:

A single layer of INVAR metal perforate (50 micron thickness) containing200 micron diameter holes positioned 500 microns apart(center-to-center) in a square grid array (supplied by Buckbee-Mears, adivision of BMC Industries, Inc.) was cleaned and micro-etched to removeundesirable dirt, oils and oxides. The pre-cleaned perforate substratethen was electroplated to provide a layer of copper metal having athickness of 9 microns.

The electrodepositable coating composition of Example 1 above waselectrophoretically applied to the electroplated substrate in anelectrodeposition bath at a temperature of 105° F. (41° C.) at 90 Voltsfor 2 minutes. The electrocoated substrate was rinsed with deionizedwater and air dried such that all holes of the perforate substrate werefree of water. The electrocoated substrate was heated to a temperatureof 350° F. (177° C.) for 30 minutes to cure the electrodepositablecoating, thereby providing a cured dielectric film thickness of 20microns.

The electrocoated substrate was then metallized. The substrate washeated to a temperature of 50° C. for a period of 30 minutes to driveoff any moisture that may have been absorbed during handling. Thesubstrate thus dried was immediately introduced into a vacuum chamberfor plasma treatment with argon ions to activate the coating surface.The substrate surface was then sputter coated with 200 angstroms ofnickel followed by sputter coating with 3000 angstroms of copper. Themetal layer thus formed was electroplated with an additional 9 micronsof copper.

The metallized substrate was cleaned and micro-etched to remove anydirt, oils or oxides from the metal surface, then electrophoreticallycoated with ELECTROIMAGE® PLUS photoresist (available from PPGIndustries, Inc.) at a temperature of 84° F. (29° C.) at 80 Volts for 90seconds. The coated substrate then was rinsed with deionized water andheated to a temperature of 250° F. (120° C.) for 6 minutes to drive offany residual solvents and/or water. A photoresist coating having a dryfilm thickness of 5 microns was obtained. The coated substrate then wasexposed to an ultraviolet light source through a phototool on each side,and developed with ELECTROIMAGE® Developer EID-523, photoresistdeveloping solution (available from PPG Industries, Inc.) to exposecopper in pre-selected areas. The exposed copper areas were etched witha cupric chloride acid etchant and the remaining photoresist strippedwith ELECTROIMAGE® stripper EID-568 photoresist stripping solution(available from PPG Industries, Inc.), thereby providing a circuitizedsubstrate.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications which are within the spirit and scopeof the invention, as defined by the appended claims.

1. A process for forming metallized vias in a substrate comprising thefollowing steps: (I) electrophoretically applying to anelectroconductive substrate an electrodepositable coating compositiononto all exposed surfaces of the substrate to form a conformaldielectric coating thereon, said electrodepositable coating compositioncomprising a resinous phase dispersed in an aqueous phase, said resinousphase comprising: (a) an ungelled, active hydrogen-containing, ionicgroup containing resin, and (b) a curing agent reactive with the activehydrogens of the resin (a), said resinous phase having a covalentlybonded halogen content of at least 1 percent by weight based on totalweight of resin solids present in said resinous phase, (II) ablating asurface of the conformal dielectric coating in a predetermined patternto expose one or more section of the substrate; (III) applying a layerof metal to all surfaces to form metallized vias in the substrate. 2.The process of claim 1, wherein the vias extend to the substratesurface.
 3. The process of claim 1, wherein the vias extend through thesubstrate.
 4. The process of claim 1, wherein the substrate comprises ametal selected from perforate copper foil, an iron-nickel alloy andcombinations thereof.
 5. The process of claim 1, wherein the conformaldielectric coating is heated at a temperature and for a time sufficientto cure the dielectric coating prior to or subsequent to step (II). 6.The process of claim 5, wherein the cured dielectric coating passesflame resistance testing in accordance with IPC-TM-350 and has adielectric constant of no more than 3.50.
 7. The process of claim 6,wherein the cured dielectric coating has a dielectric constant of nomore than 3.30.
 8. The process of claim 6, wherein the cured dielectriccoating has a dielectric constant of no more than 3.00.
 9. The processof claim 6, where in the cured dielectric coating has a dielectric lossfactor of no more than 0.01.
 10. The process of claim 1, wherein theresinous phase of the electrodepositable coating composition has acovalently bonded halogen content ranging from 1 to 50 weight percentbased on weight of total resin solids present in the resinous phase. 11.The process of claim 1, further comprising step (IV) applying a resinousphotosensitive layer to the metal layer of step (III).
 12. The processof claim 5, wherein the cured dielectric coating has a film thickness ofno more than 25 microns.
 13. A process for fabricating a circuitassembly comprising the following steps: (I) providing anelectroconductive core (II) applying electrophoretically anelectrodoepositable coating composition onto all exposed surfaces of thecore to form a conformal dielectric coating thereon, saidelectrodepositable coating composition comprising a resinous phasedispersed in an aqueous phase, said resinous phase comprising: (a) anungelled, active hydrogen-containing, ionic salt group-containing resin;and (b) a curing agent reactive with the active hydrogens of the resin(a), said resinous phase having a covalently bonded halogen content ofat least 1 percent by weight based on total weight of resin solidspresent in said resinous phase; (III) ablating a surface of theconformal dielectric coating in a predetermined pattern to expose asection of the core; (IV) applying a layer of metal to all surfaces toform metallized vias in the core; and (V) applying a resinousphotosensitive layer to the metal layer.
 14. The process of claim 13,wherein the core is a metal core selected from perforate copper foil, aniron-nickel alloy, and combinations thereof.
 15. The process of claim14, wherein the metal core comprises a perforate copper foil.
 16. Theprocess of claim 14, wherein the metal core comprises an iron-nickelalloy.
 17. The process of claim 13, wherein the resin (a) comprises apolymer derived from at least one of a polyepoxide polymer and anacrylic polymer.
 18. The process of claim 17, wherein the resin (a)comprises cationic salt groups selected from amine salt groups and/oronium salt groups.
 19. The process of claim 13, wherein the resin (a)has a covalently bonded halogen content ranging from 1 to 50 percent byweight based on total weight of resin solids present in the resin (a).20. The process of claim 19, wherein the covalently bonded halogencontent of the resin (a) is derived from a halogenated polyhydric phenolselected from at least one of chlorinated polyhydric phenol andbrominated polyhydric phenol.
 21. The process of claim 19, wherein thecovalently bonded halogen content present in the resin (a) is derivedfrom tetrabromobisphenol A.
 22. The process of claim 13, wherein thecuring agent (b) is selected from at least one of a blockedpolyisocyanate and an aminoplast resin.
 23. The process of claim 13,wherein the curing agent (b) has a covalently bonded halogen contentranging from 1 to 50 percent by weight based on total weight of resinsolids present in the curing agent (b).
 24. The process of claim 13,wherein the covalently bonded halogen content of the resinous phase ofthe electrodepositable coating composition is derived at least in partfrom a component (c) which is different from and present in addition tothe resin (a) and the curing agent (b).
 25. The process of claim 24,wherein component (c) comprises a covalently bonded halogen-containingcompound selected from the group consisting of halogenated polyolefin,halogenated phosphate ester, halogenated phenol, and mixtures thereof.26. The process of claim 13, wherein the electrodepositable coatingcomposition further comprises a rheology modifier.
 27. The process ofclaim 26, wherein the rheology modifier comprises a cationic microgeldispersion prepared by dispersing in aqueous medium a mixture of acationic polyepoxide-amine reaction product which contains amine groupsselected from the group consisting of primary amne groups, secondaryamine groups and mixtures thereof and a polyepoxide crosslinking agent,and heating said mixture to a temperature sufficient to crosslink themixture to form said cationic microgel dispersion.
 28. The process ofclaim 13, wherein prior to step (III), said conformal coating is heatedto a temperature and for a time sufficient to cure said coating.
 29. Theprocess of claim 28, wherein said cured conformal coating has adielectric constant of less than or equal to 3.50.
 30. The process ofclaim 28, wherein said cured conformal coating passes flame resistancetesting in accordance with IPC-TM-650.
 31. The process of claim 29,wherein said cured conformal coating has a dry film thickness of lessthan or equal to 25 microns.
 32. A process for fabricating a circuitassembly comprising the following steps: (I) providing anelectroconductive core; (II) providing a photoresist at predeterminedlocations on the surface of the core; (III) applying electrophoreticallyan electrodepositable coating composition over the core of step (II),wherein said coating composition is deposited electrophoretically overall surfaces of the core except at the locations having the photoresistthereon, said electrodepositable coating composition comprising aresinous phase dispersed in an aqueous phase, said resinous phasecomprising: (a) an ungelled, active hydrogen-containing, ionic saltgroup-containing resin; and (b) a curing agent reactive with the activehydrogens of the resin (a), said resinous phase having a covalentlybonded halogen content of at least 1 percent by weight based on totalweight of resin solids present in said resinous phase; (IV) curing theelectrophoretically applied coating composition to form a curedconformal dielectric layer over all surfaces of the core except at thelocations having the photoresist thereon; (V) removing said photoresistto form a circuit assembly having vias extending to said core at thelocations previously covered with said resist; and (VI) optionally,applying a layer of metal to all surfaces of the circuit assembly ofstep (V) to form metallized vias extending to said core.
 33. The processof claim 32, wherein prior to providing the photoresist in step (II) atpredetermined locations on the surface of the core, a layer of coppermetal is applied to the core.
 34. The process of claim 32, wherein thecore comprises a metal core selected from perforate copper foil,iron-nickel alloy, and combinations thereof.
 35. The process of claim34, wherein the metal core comprises an iron-nickel alloy.
 36. Theprocess of claim 34, wherein the metal core comprises perforate copperfoil.
 37. The process of claim 32, wherein the resin (a) comprises apolymer derived from at least one of a polyepoxide polymer and anacrylic polymer.
 38. The process of claim 37, wherein the resin (a)comprises cationic salt groups selected from amine salt groups and/oronium salt groups.
 39. The process of claim 32, wherein the resin (a)has a covalently bonded halogen content ranging from 1 to 50 percent byweight based on total weight of resin solids present in the resin (a).40. The process of claim 39, wherein the covalently bonded halogencontent of the resin (a) is derived from a halogenated polyhydric phenolselected from at least one of chlorinated polyhydric phenol andbrominated polyhydric phenol.
 41. The process of claim 40, wherein thecovalently bonded halogen content of the resin (a) is derived fromtetrabromobisphenol A.
 42. The process of claim 32, wherein the curingagent (b) comprises a compound selected from at least on of a blockedpolyisocyanate and an aminoplast resin.
 43. The process of claim 32,wherein the curing agent (b) has a covalently bonded halogen contentranging from 1 to 50 percent by weight based on total weight of resinsolids present in the curing agent (b).
 44. The process of claim 32,wherein the covalently bonded halogen content of the resinous phase isderived at least in part from a component (c) which is different fromand present in addition to the resin (a) and the curing agent (b). 45.The process of claim 44, wherein component (c) comprises one or morecovalently bonded halogen-containing compounds selected from the groupconsisting of halogenated polyolefin, halogenated phosphate ester,halogenated phenol, and mixtures thereof.
 46. The process of claim 32,wherein said cured conformal dielectric coating has a dielectricconstant of less than or equal to 3.50.
 47. The process of claim 46,wherein said cured conformal dielectric coating has a dielectricconstant of less than or equal to 3.30.
 48. The process of claim 46,wherein said cured conformal coating has a film thickness of less thanor equal to 25 microns.